The present invention relates to power semiconductor device structures for high-voltage and/or high-power operating conditions.
Emitter switching is a circuit configuration in which a low-voltage power transistor (typically an MOS transistor) cuts off the emitter current of a high-voltage bipolar transistor whose base is connected to a fixed potential. Thus the bipolar's V.sub.BE is controlled by varying the emitter potential in order to switch the bipolar on and off.
An example of this configuration, shown in FIG. 4, is a switched-emitter transistor structure according to commonly-owned U.S. Pat. No. 5,065,213. This U.S. Pat. No. 5,065,213, which is hereby incorporated by reference, is a pioneering disclosure of switched-emitter device structures. In the primary embodiment described in this patent, a power bipolar transistor is overlaid with a VDMOS power device. The VDMOS device is a vertical-current-flow field-effect transistor which is easily switched by an insulated gate at its surface. The drain of the VDMOS device is a buried layer which ALSO functions as the emitter of a power bipolar device. Thus the on or off state of the VDMOS changes the potential of the bipolar device's emitter (hence the name of the device). The base of the bipolar device is another buried layer (surrounding and deeper than the emitter layer), which is held at constant potential. When the VDMOS is turned on, its conduction pulls up the drain/emitter diffusion. This forward biases the base/emitter junction to turn on the bipolar. Once the bipolar is turned on it provides a lower on-resistance per unit area than would a MOS transistor of the same breakdown voltage (due to bipolar conduction and associated regeneration gain). Thus this structure provides a uniquely advantageous improvement in the tradeoff between on-resistance Ron and breakdown voltage Vmax.
Switched-emitter configurations offer several advantages:
the negative temperature coefficient of a unipolar control transistor helps protect the bipolar transistor against reverse secondary breakdown (RSBOA); PA1 the merged device combines the current and voltage carrying capacity of a bipolar transistor and the high speed of a low-voltage transistor; PA1 the merged device can be piloted directly with linear logic circuits, through the MOS gate.
The parent application proposed use of a trench FET as the control device in a switched-emitter structure. Trench FETs are particularly well-suited to this, since they provide high current density, though without the high-voltage standoff capabilities of VDMOS devices. (A VDMOS device may itself be analyzed as a composite device, i.e. a lateral DMOS device in series with a vertical JFET, which is gated by the fixed potential of the deep-body diffusion of the VDMOS. A trench FET is not a composite device in this sense.)
The present application provides a new switched emitter structure in which the emitter of the bipolar power transistor is patterned (preferably in a substantially minimum-width pattern), in a pattern which is aligned to the trenches of a trench control transistor. Thus the current density of the bipolar is maximized, since the emitter edge length per unit area is increased.
The intrinsic base width is defined by the combined doping of the buried n-type and p-type buried layers. A further advantage of the innovative merged structure is that, since large parts of the P-type buried layer are not overlain by the n+ buried layer, the extrinsic base resistance of the power bipolar is reduced (and can be further reduced, without significantly changing the characteristics of the intrinsic base, by increasing the doping per unit area of both buried layers). Thus this structure permits the parasitic resistance of both emitter and base to be reduced.
The reduced parasitic resistance of the base further implies that the bipolar gain can be specified at a fairly low value (to increase ruggedness), without an excessive voltage drop in the base (which might lead to emitter-base junction debiasing). The reduced parasitic resistance of the extrinsic base also facilitates fast turn-off and avoidance of "hot-spotting". (Non-uniform turn-off of a power transistor, especially when connected to an inductive load, can produce regions of transiently increased current density.)